A water absorbent resin (SAP/Super Absorbent Polymer) is a water swellable, water insoluble polymer gelating agent that is utilized in various applications, such as sanitation products such as paper diapers, sanitary napkins, and adult incontinence products, soil water retaining agent for agriculture and horticulture, and industrial water stopping agents. Many monomers and hydrophilic polymers have been proposed as the raw material of such water absorbent resin, but polyacrylic acid (salt)-based water absorbent resin using an acrylic acid and/or a salt thereof as a monomer is used most often from the viewpoint of performance and cost.
With higher performance of paper diapers, which are the main application of water absorbent resin, many functions (physical properties) are demanded in water absorbent resin. Specific examples of physical properties of water absorbent resin include not only a high water absorption ratio, but also gel strength, water soluble component, water absorption rate, absorption against pressure, liquid permeability, particle size distribution, urine resistance, antimicrobial property, impact resistance (damage resistance), powder fluidity, deodorizing property, resistance to coloration (degree of whiteness), low dust, and the like.
Patent Literature 1 (International Publication No. WO 2011/126079) discloses water absorbent resin powder having both liquid permeability and water absorption rate and a method of manufacturing such water absorbent resin powder.
Patent Literature 2 (International Publication No. WO 2015/030129) discloses that hydrogel is ground so as to be kneaded by using a specific shape of apparatus in one of the manufacturing steps of water absorbent resin powder, i.e., gel grinding step, to improve the absorption against pressure of the resulting water absorbent resin powder.
Patent Literature 3 (International Publication No. WO 2015/030130) discloses that hydrogel is ground so as to be kneaded by using a specific shape of apparatus in one of the manufacturing steps of water absorbent resin powder, i.e., gel grinding step, to improve the liquid permeability of the resulting water absorbent resin powder.
Patent Literature 4 (International Publication No. WO 2015/129917) discloses a water absorbent agent with reduced amount of return of an absorbing article upon actual use as defined by GCA (Gel Capillary Absorption) or the like.
Patent Literature 5 (Japanese Laid-Open Publication No. 11-60975) discloses that water absorbent resin having both a high water absorption ratio and a high water absorption rate can be manufactured by surface crosslinking to a water absorbent crosslinked polymer with —COOR content and —COOH/—COOR mole ratio (wherein R refers to a hydrogen atom, metal atom, or ammonium) within a specific range.
Patent Literature 6 (International Publication No. WO 2007/004529) discloses a method of manufacturing water absorbent resin performing multiple stages, i.e., two or more stages, of reverse phase suspension polymerization upon the manufacture of the water absorbent resin by reverse phase suspension polymerization of a water soluble ethylenic unsaturated monomer, which can manufacture water absorbent resin having both a high water absorption ratio and a high water absorption rate by adding an aminocarboxylic acid-based compound to perform a polymerization reaction in at least one of the second stage and subsequent stages.